1. Field of the Invention
The present invention relates to an oxide superconductor having a novel composition and its manufacturing method.
2. Description of the Related Art
As oxide superconductors containing Pb and Cu (hereinafter called "Pb-based Cu oxide superconductor"), Pb.sub.2 Sr.sub.2 (Ca,Y)Cu.sub.3 O.sub.8 with 2213 phase crystal structure (see R. J. Cava et al., Nature 336, pp 211, 1988), Pb.sub.2 (Sr,La).sub.2 Cu.sub.2 O.sub.6 with 2202 phase structure (see H. W. Zandbergen et al., Physica C 159, pp 81, 1989), (Pb,Sr)Sr.sub.2 (Y,Ca)Cu.sub.2 O.sub.7 (see T. Rouillon et al., Physica C 159, pp 201, 1989) or (Pb,Cu)(Sr,Ba).sub.2 (Y,Ca)Cu.sub.2 O.sub.7 (see S. Koriyama et al., Physica C 166, pp 413, 1990), both with 1212 phase structure, PbSrBa(Y,Ca)Cu.sub.3 O.sub.8 with 1213 phase structure (see A. Tokiwa et al., Physica C 161, pp 459, 1989), (Pb,Cu)(Sr,La).sub.2 CuO.sub.5 with 1201 phase structure (see S. Adachi et al., Jpn. J Appl. Phys. 29 pp. l890, 1990) have been known.
Because in these superconductors, the oxygen content in its composition is nonstoichiometric (unable to be reduced to simple integer ratio), the numbers denoting oxygen content in the above-mentioned compositional formulae indicate the ideal value. In actuality, it is assumed that the oxygen content deviates slightly from the above numbers. Of the Pb-based Cu oxide superconductors of above-mentioned composition, those containing Y are known to exhibit superconductivity characteristics even if other rare earth elements are used in place of Y.
Now when superconducting materials are used in various sensors and devices, various characteristics are required according to respective applications. For example, they must have suitable critical temperatures, must be easy to be synthesized, must have the precise structure, or must be easy to handle.
Investigating the above-mentioned Pb-based Cu oxide superconductors in respect to these viewpoints, synthesis of say a 2213-phase structure, a 2202-phase structure, and a 1213-phase structure Pb-based Cu oxide superconductors is carried out by firing material powders, a mixture of oxides, carbonates and so on, in reducing atmospheres. During firing the material powders, pinpoint control of oxygen partial pressure in the atmosphere is essential and furthermore, rapid cooling is required after firing, and therefore, such Pb-based Cu oxide superconductors are not always synthesized easily. Of the 1212-phase structures, those comprising (Pb.sub.0.5 Sr.sub.0.5) Sr.sub.2 (Y,Ca)Cu.sub.2 O.sub.7 must have their synthesis carried out in the vacuum atmosphere.
In this way, Pb-based Cu oxide superconductors of 2213 phase, 2202 phase, 1213 phase, and 1212 phase structures with the aforementioned composition are not always easy to be synthesized. Furthermore, synthesis of Pb-based Cu oxide superconductors with the above-mentioned structures is carried out in either a reducing or a vacuum atmosphere as described above. In general, the melting point of oxides is lowered in such atmosphere. Consequently, the firing temperature must be lower than that in oxidizing atmospheres, resulting in a problem that Pb-based Cu oxide superconductors obtained are difficult to be densified.
Of Pb-based Cu oxide superconductors of the 1201-phase structure and the 1212-phase structure, those comprising (Pb,Cu)(Sr,Ba).sub.2 (Y,Ca)Cu.sub.2 O.sub.7 can be synthesized in an oxidizing atmosphere and the problem as mentioned above would not occur. However, Pb-based Cu oxide superconductors of this composition contain a high content of Pb, a harmful element, requiring careful handling. This constitutes a disadvantage to produce a practical Pb-based Cu oxide superconductors.